Field of the Disclosure
The present disclosure relates to a process to treat tailings streams produced in mining operations to provide a deposit utilizing delayed gelation.
Description of Related Art
Tailings, as a general term, refers to byproducts from mining operations and processing of mined materials in which a valuable material such as a metal, mineral, coal, and the like, is separated, for example, extracted, from a mined material, that is, material which has been removed from the earth. Tailings typically comprise clay and optionally sand and/or rock. Tailings further comprise water. Water is used in combination with mechanical and/or chemical processes for removing the valuable material from the mined material. Mining operations include those for precious metals, base metals, ores, clays and coal. In addition, mining operations include recovery of bitumen from oil sands.
Tailings treatment and disposal are major issues for mining operations. Water recovery from the tailings for re-use in extraction processes and transportation is often desired. Tailings solids, such as clay and sand, as well as other solid materials such as rock are generally sent to a storage facility or disposal area local to the mining operation. Management of such storage facilities or disposal areas is an enormous task.
Storage or disposal of tailings involves construction of a facility that is safe for storage (including permanent storage), sufficiently large and stable to contain the tailings within the facility, and protecting the local environment. It may be desirable to access water from the tailings storage facility for use in mining operations such as extracting and other treatments.
Various tailings streams are produced in extraction processes. A tailings stream is an aqueous stream (slurry, suspension) containing components requiring further treatment, which may include extraction of valuable material or solids removal and/or purification to enable recycle of the water content of the tailings stream. Some tailings streams will be deposited in a tailings pond for long periods of time, including permanently. Coarse solids may settle quickly. The top layer of the pond may clarify with time to make water that is suitable for re-use in the extraction process. A layer may comprise water and fine solids, which solids settle very slowly. This layer may ultimately become mature fine tailings (MFT).
MFT is a stable composite slurry comprising one or more of clay, sand, silt, water and optionally rock. MFT has little strength, no vegetative potential and may be toxic to animal life, so it must be confined and prevented from contaminating water supplies. Typically, several years of settling time are required to make MFT, which may have little additional settling or consolidation occurring for decades.
MFT ponds pose an environmental concern. For example, the Energy Resources Conservation Board of Alberta (ERCB) has issued Directive 074, which mandates a reduction of MFT ponds and the formation of trafficable deposits for MFT produced in mining and extraction of bitumen from oil sands by all oil sands operators.
Moffett disclosed, in US 2010/0104744 A1, a process to treat tailings streams with a silicate source and an activator. The silicate source is an alkali metal silicate, polysilicate microgel, or combinations thereof. The activator may be an acid, alkaline earth metal salt, aluminum salt, organic ester, dialdehyde, organic carbonate, organic phosphate, amide, or a combination thereof.
Alkali metal silicate solutions are distinct from colloidal silica sols by their ratio of silica to metal oxide (SiO2:M2O). For example, solutions of sodium silicate have SiO2:Na2O of less than 4:1, as disclosed by Iler, “The Chemistry of Silica”, Wiley Interscience (1979), page 116. Iler further recited that “silicate solutions of higher ratios were not available.”
Moffett disclosed in U.S. patent application Ser. No. 13/329,375, filed Dec. 19, 2011, a process to treat tailings streams with a gelling agent and an activator. The gelling agent is selected from the group consisting of colloidal silica, aluminum-modified colloidal silica, de-ionized colloidal silica, polysiloxane, siliconate, acrylamide, acrylate, urethane, phenoplast, aminoplast, vinyl ester-styrene, polyester-styrene, furfuryl alcohol-based furol polymer, epoxy, vulcanized oil, lignin, lignosulfonate, lignosulfite, montan wax, polyvinyl pyrrolidone, and combinations of two or more thereof. The activator can be any compound or mixture of compounds that will initiate gelation.
An important aspect of tailings management is consolidation of the tailings solids—that is, to reduce the water content of the deposited tailings, for example to minimize storage space required upon disposal.
Placement of the treated tailings on a sloped surface is one preferred deposition technique. Ideally, the treated tailings are deposited on the highest point or a sloped deposition field and allowed to traverse the sloped field and gel on the same slope as the field. It is desired that the gel formation occur slowly as the deposit flows down the hill to avoid a large build-up of the treated solids at the top or the first section of the slope. Previous attempts to alter gel formation delay times include decreasing the concentration of the silicate source, decreasing the concentration of the activator, or the use of an ester as an activator. While these methods are somewhat effective, the dewatering and strengths of the tailings can also be affected. While there have been many advances in the treatment of tailings, there remains a need to improve one or more of de-watering (less water in the tailings), consolidation (reduction of volume of the tailings), and strengthening of the tailings using a delayed gelation process. The present invention meets these needs.